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Investments - Net Investment Income and Investment Gains (Losses) (Parenthetical) (Detail) - USD ($)
$ in Millions
3 Months Ended
Mar. 31, 2019
Mar. 31, 2018
Net Investment Income [Line Items]    
Net unrealized gains (losses) on trading securities $ 44 $ (25)
Nonredeemable Preferred Stock [Member]    
Net Investment Income [Line Items]    
Net realized gains (losses) on trading securities 42 (15)
Fixed Maturity Securities [Member]    
Net Investment Income [Line Items]    
Gross realized gains on available-for-sale securities 36 69
Gross realized losses on available-for-sale securities $ 42 $ 51